The heart of any ultrasound (imaging) system is the transducer which converts electrical energy in acoustic energy and back. Traditionally these transducers are made from piezoelectric crystals arranged in linear (1-D) transducer arrays, and operating at frequencies up to 10 MHz. However, the trend towards matrix (2-D) transducer arrays and the drive towards miniaturization to integrate ultrasound (imaging) functionality into catheters and guide wires has resulted in the development of so called capacitive micromachined ultrasound transducers (CMUT) cells. These CMUT cells can be placed or fabricated on top of an ASIC (Application Specific IC) containing the driver electronics and signal processing. This will result in significantly reduced assembly costs and the smallest possible form factor.
Preferably the CMUT cells are fabricated in a separate dedicated technology which is optimized for performance and placed on top of the ASIC. An important question which then needs to be addressed is how the CMUT cells are to be connected to the ASIC. One solution is to use a through-wafer via device. The through-wafer via device, manufactured with a suitable through-wafer via hole technology, can then be used to connect the CMUT cells on the front surface of the wafer to contacts on the backside surface of the wafer. In this way the CMUT cells can be “flip-chipped” (e.g. by solder bumping) to the ASIC.
US 2008/0203556 A1 discloses a through-wafer interconnect and a method for fabricating the same. The method starts with a conductive wafer to form a patterned trench by removing material of the conductive wafer. The patterned trench extends in depth from the front side to the backside of the wafer, and has an annular opening generally dividing the conductive wafer into an inner portion and an outer portion whereby the inner portion of the conductive wafer is insulated from the outer portion and serves as a through-wafer conductor. A dielectric material is formed or added into the patterned trench mechanical to support and electrically insulate the through-wafer conductor.
However, this through-wafer interconnect and method for fabricating the same require the trench to be filled with a mechanically strong and electrically isolating dielectric which needs to be able to withstand high temperature processing steps required for the fabrication of the CMUT cells.
There is a need to further improve such through-wafer interconnect and method for fabricating the same.